Evaluation of competitive and synergistic effects of potential determining ions on interfacial tension reduction and wettability alteration in carbonate oil reservoirs

This study examines the effects of divalent ions (Ca 2+ , Mg 2+ , and SO 4 2- ) in seawater (SW) on rock-fluid and fluid-fluid interactions in carbonate oil reservoirs. Contact angle experiments, interfacial tension (IFT) measurements, and micromodel flooding tests were conducted to evaluate the roles of divalent ions in IFT reduction and wettability alteration mechanisms. The results reveal that Mg 2+ , possessing the highest hydration energy among the tested ions, has the greatest impact on reducing IFT, achieving a minimum value of 11.13 mN/m at 50°C for the 2Ca2Mg0S brine. While SO 4 2- alone reduces IFT, its interaction with Ca 2+ and Mg 2+ results in an increase in IFT. Contact angle measurements demonstrate that brines containing SO 4 2- shift the carbonate rock's wettability toward a more water-wet state, with 2Ca2Mg2S achieving the most significant reduction, decreasing the contact angle to 48.37°. Micromodel flooding experiments indicate that IFT reduction dominates oil recovery after a 2-hour shut-in period, whereas wettability alteration becomes the primary recovery mechanism after 24 and 48 h. Recovery factors increase from 50.92 % to 53.37 % for 2Ca2Mg0S and from 47.24 % to 56.25 % for 2Ca0Mg2S after a 48-hour shut-in. These findings highlight the complex effects of divalent ions in enhancing oil recovery and provide valuable insights for optimizing brine compositions in enhanced oil recovery (EOR) process. • Mg²⁺ shows the strongest effect on IFT reduction among divalent ions. • SO₄²⁻ shifts wettability to water-wet, reducing the contact angle to 48.37°. • IFT reduction dominates initially; wettability alteration prevailed in later stages. • SO₄²⁻-cation interactions increase IFT due to competitive electrostatic effects. • Dynamic IFT explains by coupled diffusion and EDL expansion models.